This invention relates to a process for the production of medium density, fire retardant decorative molded foams in which the in-mold time is significantly reduced. The invention also relates to the fire retardant composition employed in this process and to foams produced by this process.
Polyurethane foams are used for a wide variety of applications, such as thermal insulation, packaging, upholstery, carpet underlay, automobile dashboards, building materials, and structural material. An important factor to be considered in employing polyurethane foams is the ability of such materials to resist ignition, or once ignited, to be self-extinguishing after the ignition source is removed. This factor becomes even more important if the polymeric material is to be used within a confined space or in outdoor applications in locations that are fire-prone. These polyurethane foams can even be used, for example, as roofing materials in fire-prone areas.
As those skilled in the art are aware, the most common method of decreasing the flammability of polyurethane foams is by incorporating a flame retarding agent, such as a halogen- or phosphorus-containing compound, into the foam formulation. Although such compounds provide some improvement in the flame retardation properties, relatively large quantities of these agents may have to be employed to obtain satisfactory results. In general, incorporating relatively high amounts of flame retardants into the polyurethane foam reduces the overall physical property levels of the polyurethane foam.
For many years, the dominant blowing agents used to expand polyurethane foam had been the cholofluorocarbons. These blowing agents were phased out after having been determined to pose a threat to stratospheric ozone. After the cholofluorocarbons were phased out, the most common class of blowing agents became the hydrogenated chlorofluorocarbons. Although these are considered to be somewhat more environmentally friendly expansion agents, the hydrogenated chlorofluorocarbons still contain some chlorine. The chlorine atoms of hydrogenated chlorofluorocarbons are stable at altitudes under the stratosphere, and thus have a lower ozone-depleting potential (“ODP”). However, because of the hydrogenated chlorofluorocarbons still have a small ODP, they have also been mandated for eventual phase out. Water and/or carbon dioxide are rapidly becoming the blowing agents of choice for polyurethane foam manufacturers.
As known to those skilled in the art, polyurethane foams can be made using trimethylolpropane-based polyols (See e.g., U.S. Pat. Nos. 6,319,962, 4,690,954 and 4,407,981). Although there are some polyurethane foams available that pass the ASTM E-84 Tunnel Test “Standard Test Method for Surface Burning Characteristics of Building Materials” (ASTM International) with a Class I rating (U.S. Pat. Nos. 4,797,428 and 4,940,632), these foams use the alternative chlorofluorocarbon/hydrogenated chlorofluorocarbon blowing agents in combination with highly loaded polyester polyol blends and liquid flame retardants or have high flame retardant filler loadings, including phosphorus-based materials, in combination with trimethylolpropane-based polyols to produce the desired end result. These polyester-containing foams tend to reduce long term hydrolytic and “creep” stability and thus become a problem for applications outside of insulation-type foams.
U.S. Pat. No. 5,086,084 describes a foamed polymeric material suitable as a wood substitute, made of a continuous phase of polyurethane having solid polyvinyl chloride particles dispersed therein. The wood-like material of this patent contains about 100 parts of a foamable urethane, and 10 to 50 parts polyvinyl chloride (PVC) particles having a particle size below 200 μm. This material has a microcellular structure with cells on the order of 0.1 mm in average diameter or less. The walls are said to be made of a matrix of polyurethane reinforced with PVC particles. There is, however, no mention of the heat performance properties of this wood substitute and de-mold times (i.e., time after which the molded article may be removed from the mold) of from five to ten minutes are disclosed.
Therefore, despite the abundance of disclosed processes to obtain flame retardant foams, polyurethane foam manufacturers remain interested in a foam that is solely water- or carbon dioxide-blown; that satisfies the burning brand test ASTM E-108 with a Class A rating. A flame retardant combination that minimizes the amount of halogen-containing compounds would also be highly desirable from an environmental perspective.
Thus, the development of such flame retardant polyurethane foam would be very desirable. Because of environmental concerns, it would be also be desirable for such a foam use non-chlorofluorocarbon/hydrogenated chlorofluorocarbon-containing blowing agents, such as water and/or carbon dioxide.
The paper titled “Ammonium Polyphosphate-Aluminum Trihydroxide Antagonism in Fire Retarded Butadiene-Styrene Block Copolymer” by A. Castrovinci et al in European Polymer Journal, (2005), 41(9), 2023-2033, discusses the effect of aluminum trihydroxide (ATH) on the surface protection from fire provided by ammonium polyphosphate (APP) to a styrene butadiene rubber (SBR). It was necessary to add a significantly higher amount of ATH than APP to achieve comparable results, i.e. 60 wt. % of ATH vs. 10-12 wt. % of APP. In addition, the substitution of 1 wt. % of ATH for APP in an SBR containing 12 wt. % of APP showed an antagonistic effect. This is explained by Castovinci et al by the interaction between SBR, APP and ATH in which aluminum phosphates form on heating APP in SBR, and these aluminum phosphates negatively affect the surface protection that the APP provides to the SBR.
The interaction between two fire retardants was studied in “Structural and Thermal Interpretation of the Synergy and Interactions Between the Fire Retardants Magnesium Hydroxide and Zinc Borate” by A. Genovese et al, Polymer Degradation and Stability, (2007), 92(1), 2-13. The fire performance of a polyolefin with a magnesium hydroxide fire retardant reduces the heat release rate through absorption of heat during conversion to magnesium oxide. Zinc borates which undergo dehydration with increasing temperatures also increased fire performance of a polyolefin. Various structural changes were seen in the zinc borates. Endothermic transitions occurred in zinc borates, and 2ZnO.3B2O3.3H2O underwent an exothermic crystalline transition at a high temperature. In addition, magnesium orthoborate (3MgO.B2O3), a new crystalline phase, and some zinc oxide (ZnO), formed on reaction of magnesium oxide with zinc borate (2ZnO.3B2O3.3H2O) at temperatures greater than 500° C. Thus, it appears that there is a synergy that results from the combination of magnesium hydroxide and zinc borates as flame retardants for polyolefins.
Flame resistant, thermoplastic polyurethane elastomers and processes for their preparation are disclosed in U.S. Pat. No. 4,748,195. The flame retardant package in these TPUs is (a) a compound selected from the group consisting of antimony trioxide, zinc borate and mixtures thereof, (b) a chlorinated polyethylene, and (c) a brominated aromatic compound selected from the group consisting of polytetrabromo-bis(phenol)-A-glycidyl ether, polytribromostyrene and polytetrabromo-bis(phenol)-A-carbonate. Polyurethane foams are not mentioned in this disclosure. This disclosure is also silent with respect to de-mold times required for producing molded polyurethane elastomers from the disclosed flame retardant package.
EP Application 0,512,629 discloses the usefulness of zinc borate in combination with encapsulated ammonium polyphosphate in thermoplastic urethanes. It also discloses that the solid elastomer compositions can achieve V0 rating in a UL94 vertical burn test. The flame retardant combination must contain, in addition to zinc borate and a “carbonific” (polyhdric char-forming) compound such as pentaerythritol, a large excess of ammonium polyphosphate comprising from 30 to 50% of the filled polyurethane. These materials have densities of 65 to 100 pcf making them less attractive as construction materials from a practical and economical perspective. This disclosure is silent with respect to de-mold times for articles produced with the disclosed flame retardant combination.
Zinc borates and their use as fire-retardants in halogen-containing and halogen-free polymers are described in “Recent Advances in the use of Borates as Fire Retardants” in the journal “Recent Advances Flame Retardant Polymeric Materials”, (RAFMFH), 1995, Vol. 6, pp. 239-247. Advantages of zinc borates include its ability to act as a smoke suppressant, flame retardant, afterglow suppressant, char promoter, anti-arcing agent, and improves oil resistance, and inhibits plate out in polymers containing siloxanes.
U.S. application Ser. No. 12/231,153 (PO-9133) describes polyurethane foams made with improved flame retardant foam systems that are produced with water and/or carbon dioxide as the blowing agent, i.e., which are free of halogenated flame retardants. The flame retardant described in this application is a composition which includes both ammonium polyphosphate and zinc borate. Advantages of the foams described in this application include: (1) medium density (10-30 pounds per cubic foot (pcf)) making them suitable for construction materials; (2) flame resistance allowing them to be used in fire-prone areas; (3) flame retardants which do not include halogen-containing compounds; and (4) such low amounts of flame retardant that they are flexible enough for demanding applications.
The foams described in U.S. Ser. No. 12/231,153 have the disadvantage of a long cure time unless relatively large amounts of catalyst are included in the reaction mixture. This long cure time makes it necessary to allow the polyurethane-forming composition to remain in the mold for long periods of time thus reducing production capability.
U.S. Pat. No. 4,467,056 describes an ammonium polyphosphate flame retardant which is encased in a hardened water-insoluble polycondensation product of melamine and formaldehyde. This flame retardant is taught to be useful as a flame retardant in plastics, particularly, polyurethane foams. However, because the ammonium polyphosphate is encapsulated using an aqueous methanol solution of melamine and formaldehyde, removal of all water from the encapsulated ammonium polyphosphate can not be ensured. The presence of such residual water in a foam-forming mixture can be expected to affect the density of the product foam. Because the amount of residual water would be expected to vary, control of foam density made with ammonium polyphosphate encapsulated in this manner would be difficult. U.S. Pat. No. 4,467,056 does not, however, disclose anything with respect to adaptation of foam-forming systems in which the disclosed flame retardant is employed. This patent is also silent with respect to de-mold times for molded polyurethane foams.
U.S. Pat. No. 5,534,291 discloses a process for producing melamine-coated ammonium polyphosphate particles. This patent does not, however, teach that use of the disclosed melamine-coated ammonium polyphosphate would have any effect upon the rate of reaction of a system into which such a flame retardant component is incorporated.
U.S. Pat. No. 5,945,467 describes flame-retardant thermosetting resin compositions in which melamine-coated ammonium polyphosphate particles and/or water-insoluble ammonium polyphosphate particles is/are included in a thermosetting resin. This patent does not, however, teach or suggest that use of the melamine-coated ammonium polyphosphate particles would have any effect upon rate of reaction.
It would therefore be advantageous to develop a process for producing flame retardant medium density polyurethane foams without the use of any halogenated flame retardant or blowing agent or a large amount of catalyst in which the rate of cure is substantially reduced, thereby enabling the production of molded articles with significantly shorter de-mold times than current systems.